To meet an output voltage or output power requirement, multiple rechargeable battery cells are generally connected in series to form a battery pack. The multiple battery cells in the battery pack should have a same energy level in an ideal situation. On one hand, for a battery cell produced in practice, a self-discharge phenomenon exists when the battery cell is not used, causing stored energy leakage in the battery cell. On the other hand, due to aging materials, an energy storage capacity of the rechargeable battery cell decreases during reuse. Degrees of stored energy leakage and degrees of energy storage capacity decrease of different battery cells are related to internal resistances of the battery cells, an ambient temperature difference, and inconsistency occurring in a production process. For the multiple battery cells connected in series, even if normal energy consumptions caused by working currents are the same, energy levels of the battery cells in the battery pack are different because of stored energy leakage and energy storage capacity decrease.
The rechargeable battery cell needs to operate in a proper energy level interval, and an excessively low or an excessively high energy level may cause damage to the battery cell. For example, when the battery pack is discharged, a battery cell with a lowest energy level reaches a lower limit of an energy interval first, leading to end of normal discharge of the battery pack. When the battery pack is charged, a battery cell with a highest energy level reaches an upper limit of the energy interval first, leading to end of normal charge of the battery pack. Therefore, when energy levels of the multiple battery cells in the battery pack are different, if no corresponding energy balancing measure is taken, the battery pack as a whole cannot be fully charged during energy charge and cannot be fully discharged during energy discharge. Consequently, an actual capacity available for the battery pack is reduced. To make the most of energy stored in each battery cell in the battery pack, it is necessary to perform energy balancing on the multiple battery cells connected in series.
A purpose of energy balancing is to ensure a consistent energy level for the battery cells connected in series in the battery pack. A current energy balancing method is active balancing. That is, energy of a battery cell with a high energy level is transferred to a battery cell with a low energy level, so that an energy level of each battery cell approaches an average value in the battery pack. Alternatively, from an energy supply outside the battery pack is supplemented to a battery cell with a relatively low energy level in the battery pack, so that an energy level of each battery cell approaches a highest value in the battery pack.
In the prior art, energy balancing solutions implemented according to the active balancing idea include a balancing circuit based on a Buck-Boost converter, a balancing circuit based on multiple full-bridge converters, and the like. As shown in FIG. 1, FIG. 1 shows a balancing circuit based on a Buck-Boost converter. There are N battery cells in FIG. 1, which are B1, B2, . . . , BN, respectively. To implement energy balancing, a full-control switch component needs to be configured for each battery cell. In this case, 2N−2 full-control switch components need to be configured for the N battery cells, where N is not less than 2. As shown in FIG. 2, FIG. 2 shows a balancing circuit based on multiple full-bridge converters. There are N battery cells in FIG. 2. However, 4N full-control switch components are required to control energy flow, so as to implement energy balancing.
It can be learned from FIG. 1 and FIG. 2 that, although the foregoing solutions can implement energy balancing, there is a relatively large quantity of full-control switch components (sw shown in FIG. 1 and FIG. 2 represents a full-control switch component) in the foregoing solutions, a drive system is complex, and costs are relatively high.